THE CLASSIFICATION OF FOSSIL PLANTS 
ranges rose, among them the Alps, the Himalayas, the 
Pyrenees, the Rockies, the Apennines, and the Andes. 
The climate gradually became progressively colder and 
immense ice-caps formed on the northern parts of the 
continents of North America, Europe, and Asia. They 
advanced and retreated several times, with alternating 
warming and cooling of the atmosphere, until the modern 
era in which we are now living dawned. Man, it is be¬ 
lieved, first appeared at some time in the Tertiary period. 
It is not known whether we are living today in a post¬ 
glacial or in an inter-glacial period. We may have passed 
out of the ice age entirely, as some geologists think, and 
in the course of another 100,000 years the climate may 
become universally warm again. Or, the glaciers and ice- 
sheets may come back again in the northern hemisphere; 
and the land where our present centers of civilization now 
stand, may in 10,000 years be covered with masses of ice 
hundreds of feet thick. 
Plant life depends on a number of what we call ecologi¬ 
cal or environmental factors. Ecology is a word closely 
related to economics. Just as human life is dependent 
largely on economic factors, which consist of food sup¬ 
plies, shelter, clothing, and many other necessaries which 
even a primitive man must have; so animals are dependent 
on food, climate, and habitat, such as water, forest, 
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